Of course it's just game mechanics, but I never really understood why none of the soldiers aboard a transport unit like an APC, T-Copter or Lander even tries to return fire when the enemies starts shooting at them...
Get the parts and instructions to build your own papercraft Advance Wars Orange Star Lander (and all my other papercraft models too! ;o) from my free papercraft webpage: [link]
Even from a balance perspective I don't think it'd be an issue. Most of the units that can do decent damage to a Lander or APC wouldn't be particularly vulnerable to infantry fire. A Recon unit still has a lot of advantages against a loaded APC because of it's movement and cheaper cost. The only real difference would be that you couldn't as easily attack one with Infantry without taking damage, and I don't think that's a bad thing.
Exactly, I guess this way is better for the balance between the units in the game, but in real life I would imagine they would definitely at least *try* to return fire at whatever is shooting at them! ;o)